The invention pertains generally to improved systems for generating displays from stored video data, and more particularly to spiral scan display apparatus capable of high speed generation of distortion free visual presentations.
It has become common practice to produce visual presentations on a display device in accordance with information in digital form supplied by a computer or other source of digital signals. Various techniques for converting the digital data to a visual form are known. The techniques are implemented in common display apparatus by spiral scan as well as raster scan methods. One common type of display apparatus employs a cathode ray tube as the display device. Several known techniques are used in connection with cathode ray tube display apparatus for obtaining display presentations of sufficient persistence to permit visual observation. One such technique is to utilize long persistence phosphors in the cathode ray tube so that the presentation remains visible for an interval of time after scanning by the electron beam. This technique is not suitable for displays which must be updated at very frequent intervals or for displays in which significant information is contained in the relative brightness levels of points on the display area.
A second technique involves continually refreshing the visual presentation by repeatedly displaying the video data in such rapid succession that a constant visual appearance is provided. This technique permits very frequent updating of the presentation, and also permits information to be conveyed in the form of brightness levels and variations on the display area.
In order to produce a satisfactory display by the latter technique, the entire display area must be scanned at very frequent intervals. In general, careful synchronization of the video input data with position of the electron beam (or other display writing means) is required. As a practical matter, such synchronization involves critical timing relationships and predictable positioning of the electron beam. Problems in implementing these criteria are compounded as the updating speed and display content and accuracy requirements are increased.
In a digitally driven display of the spiral scan type, the display writing means (electron beam in a cathode ray tube) is deflected around a circular path whose radius is periodically incremented so as to scan a display area. One of the problems encountered in achieving very high speed operation in display apparatus of this type is that switching transients are caused as the beam path radius is incremented. The radius of the beam path must be switched very rapidly in order to meet the time requirements for completing a scan of the display area. It has been found that switching of the beam path radius is frequently followed by an interval of transient oscillation of beam position about its intended path. This is reflected on the generated display as an area of distortion.
The applicant's invention is a method and apparatus for overcoming this problem by suppressing any visual presentation during intervals when undesired transients exist. The applicant has discovered that this may be accomplished primarily through a unique reordering of the stored video data in conjunction with minor changes and additions to existing display apparatus.